Annual CO2 emissions produced by the cement industry are of the order of 4 Gt. There are therefore methods being made to reduce the CO2 emissions per metric ton of cement, by measures such as fuels from biomass, residual heat recovery, substitution of cement clinker by pozzolans, or the deposition of the CO2 from smoke gases. A particularly effective measure here is the reduction of the clinker factor, by replacing part of the clinker with pozzolanic substances, such as slag sand, flyash, natural pozzolans, or limestone. In order to maintain an overall clinker factor of currently 76 (wwwb.wbcsdcement.or), the demand for clinker substitutes in the case of annual cement production of around 4 Gt is around 1 Gt. Against the background of the increasing production of cement worldwide, moreover, the demand for clinker substitutes will increase further.
The decision in favor of a binder substitute is made according to the regional availability of the material and the associated costs of acquiring it and the logistics. In the cement, furthermore, any clinker substitute (limestone, slag sand, flyash, pozzolan) makes a specific contribution to hydration reactions and hence to the development of strength.
Depending on the use of the cement and on the regulations for binder composition and on the concrete standards, levels of clinker substitution in Europe, apart from the secondary constituents (0-5%), are 5-95% (CEM II A/B, CEM IIIC). Limestone in particular, however, can usually be added only up to about 20% without a critical loss of binder performance. Other sets of standards, with national validity, are oriented not on the binder composition but instead solely on the functionality of the binder, and allow even higher levels of addition of pozzolanic additives (e.g., ASTM C595 , SNI 15-0302-2004), or have no regulations at all regarding cement composition (ASTM C1157).
Another clinker substitute, though having been little utilized up to now, constitutes calcined clays, which can be used as a clinker replacement and additive in cement and concrete. DE 10 2011 014 498 B4 describes in more detail a method for producing a clinker replacement from calcined clay.
Clay minerals are phyllosilicates—sheet or layered silicates—which are characterized by a stacked alternation of silicon-rich, aluminum-rich, or silicon-rich and aluminum-rich layers with interlayers of hydroxide, alkali metals, or water. Inexpensive clays, moreover, often have impurities, as a result of elevated iron levels, for example. Depending on the manifestation of the iron, elevated iron levels in particular may result, on calcining, in a typical red discoloration, which is apparent to start with in the calcined clay, but then also in the cement and concrete when the clay is used as a clinker substitute, this discoloration arising from the reddening trivalent iron.
In view of the costs of high-purity clays, simple inexpensive grades are the only ones contemplated as a clinker substitute of binders. Clay manifestations—especially those featuring low purity—are widespread. In order to correct the red discoloration, DE 10 2011 014 498 B4 proposes a special cooling process under reducing conditions, or the injection of oil into a cooler in order to generate a reducing atmosphere. In this reducing atmosphere, up to ⅓ of the reddening trivalent iron in the hematite is reduced, and the gray iron-iron spinel (magnetite) is formed. The gray color can be accorded particular significance in the context of acceptance of the cement by the end customer.
In the calcining of clay, which is carried out for example in a rotary kiln, a suspension-type heat exchanger, a fluidized bed, or else in a multitier furnace, the clay minerals first of all give up surface water and then lose structural water. A consequence of this giving-up of water is the phase transition of the crystalline clay minerals into X-ray-amorphous meta-clay minerals. On further heating (500-1250° C.) of the clay minerals, the melting of the meta-clay minerals is accompanied by formation either of aluminosilicate glasses or else, in the case of an appropriate chemical/mineralogical composition, of high-temperature phases such as mullite or cristobalite. The flame temperature of an internal firing is difficult to regulate in the low-temperature range. For this reason, external firing chambers are usually used for calcining clays, and the hot gas is brought to the desired calcining operation temperature using cooler external air. An inherent drawback of this method is the high level of excess air, with the risk of extensive oxidation of the iron compounds from the raw clay materials.
The pozzolanic properties of the calcined clays are favorable for use as a clinker substitute. A further advantageous solution would be to combine calcined clays with those materials—flyashes, for example—which enhance other qualities of the cement (e.g. the workability, the water demand, or the hydraulic reactivity) or which permit a higher clinker substitution rate. It is known practice to generate composite cements with calcined clay and flyash by joint grinding or by mixing in a mixing unit. In both cases, however, there is a need not only for a calcining unit but also for further storage and associated conveying and mixing equipment for all the components of the blended cement.
A need exists, therefore, for methods for producing a pozzolanic or latent hydraulic cement clinker substitute that is more cost effective and that can be rapidly produced.
It is an object of the invention, therefore, to specify a method for producing a pozzolanic or latent hydraulic cement clinker substitute that is notable for more cost effective and more rapid production.